In recent years, major advances have been made in small particle technology. Many industries are particularly concerned with the characteristics and effects of small particles.
In the paint industry, particle size may determine the opacity of pigments, and the durability, consistency, dispersibility, etc. of paint. In the cement industry, particle size is important in determining permeability, rate of hydration, strength, etc. of concrete. In the ceramics field, particle size is important in adjusting the porosity, strength, plasticity, effect on sintering, etc. of the ceramic products. Other industries where particle size is important, if not critical, include power metallurgy, pulp and paper, soaps and synthetic detergents, catalysts, air pollution control, aerosol technology, and health, among others. In these industries, products are produced in which product size is critical for the intended performance of the particular product.
In view of the wide spread use of small particles in many different technologies, it is important to be able to accurately determine particle size. Some of the better known methods for determining particle size based on physical properties of the particles are sedimentation, permeability, light scattering, adsorption, diffusion and sieving.
The present invention relates to an automated wet sieving apparatus and process for analysis of particles below about 100.mu. in diameter. Of particular interest are particles below about 44.mu. in diameter.
Two published articles co-authored by applicants represent the closest prior art known to applicants. In R. D. Desai, B. Toth and R. Somkaite, "Particle Size Analysis Of Chlorhydrol By Wet Sieving," Drug & Cosmetic Industry, December 1972, p. 50, one type of wet sieving apparatus and process is disclosed. Chlorhydrol, a registered trademark of Reheis Chemical Company for aluminum chlorhydrate, is a prime active ingredient in aerosol antiperspirants. Because of the use of relatively high concentrations of this highly deliquescent aluminum salt, aerosol valves through which this product is dispensed tend to clog. An Analyt-3 dry and aqueous wet sieving apparatus was extensively modified to permit non-aqueous wet sieving. A sample of Chlorhydrol was placed on a selected sieve and was subjected to vertical oscillations produced by an electromagnet. A varistaltic pump controlled the flow of wetting liquid through a spray nozzle which wetted the particles until the liquid issuing from the sieve was clear. The residue remaining on the sieve was then weighed.
The Analyt-3 instrument was unsuccessfully modified so as to use 3 inch standard sieves (44 .mu. and smaller) instead of the 8 inch standard sieves. Two major factors were regulated during the sieving process: (1) the vertical oscillations of the vibrator and (2) the flow rate of the wetting liquid. Even at the lowest vibrating amplitude, the Chlorhydrol particles did not pass through the sieve, primarily because of the low density of the sample particles. Consequently, the hindered particles allowed the wetting liquid to flow uniformly through the sieve, which resulted in the accumulation of liquid to the point of overflow.
In their second publication, R. Somkaite, R. D. Desai, and B. Toth, "Particle Size Analysis Of Chlorhydrol," Drug & Cosmetic Industry, November 1974, p. 54, the Analyt-3 was successfully modified to accommodate the 3 inch standard sieves. The bottom pan of the Analyt-3 instrument was modified so that the vibration was uniformly distributed to permit satisfactory sievability when a 3 inch sieve rests on it. The same procedure was then followed as for eight inch standard sieving. The precision of the method using the modified Analyt-3 instrument for wet microsieving of Chlorhydrol, using 10 .mu., 15 .mu. and 44 .mu., 3 inch standard size sieves was reported as considerably improved over their first publication. The sieves used in the modified apparatus still showed a tendency to become blinded. Additionally, the wetting liquid is directed at the same portions of the sieve when using a particular spray nozzle, rather than being distributed over substantially the entire surface of the sieve, as would be possible by swirling the wetting liquid as it passes over the sieve.
Two U.S. patents issued to Pitchford, U.S. Pat. Nos. 3,167,259 and 3,167,503, deal with the problem of blinding of screens used in particle separation. Particles to be separated are introduced into a particle receiving chamber having screens arranged at opposite ends of the chamber. Air is pulsed alternately through the chamber so that the screen at the inlet end is cleaned and the particles within the chamber are blasted against the screen at the outlet end of the chamber. In the alternate pulse, the direction of air is reversed so that the original outlet end becomes the inlet end for the air. Thus, the screen at the former outlet end is cleaned and the particles within the chamber are blasted against the screen at the former inlet end. This process is repeated until the desired degree of separation is attained. The air pulsed alternately into the chamber may be produced by positive pressure on the inlet side or by a negative pressure on the outlet side. There is no disclosure of alternating positive pressure with negative pressure as is done in the process and apparatus of the present invention.